Email address + manage subscriptions + server API update.

[ADDED]     Email address configuration for identification.
[ADDED]     Encode email address for use as opaque identifier.
[ADDED]     Manage subscriptions when multiple are active.
[UPDATED]   Use PearlOverlay instead of PearlAlert for activities.
[UPDATED]   Handle cloud key changes.
[UPDATED]   Fetch latest state from server on startup.
[UPDATED]   Restore purchases on long-press.
[UPDATED]   Improved server API & unhardcoded debug strings + handle failures.